U.S. Pat. No. 5,374,564 discloses a method of fabricating thin films of semiconductor materials that includes the following steps:
(1) bombarding one face of a substrate with ions in order to implant those ions in a concentration sufficient to create in the substrate a layer of gas microbubbles forming microcavities defining a weakened layer;
(2) bringing this face of the substrate into intimate contact with a stiffener; and
(3) obtaining cleavage at the level of the microcavity layer by the application of heat treatment.
In the process described above, the ions implanted in step 1 are hydrogen ions, but it is indicated that light ions of helium type or of other rare gases may also be used. As for the substrate, in the examples concerned it is formed of silicon, but it is indicated that it may also be a semiconductor from group IV of the periodic table of the elements, such as germanium, silicon carbide or silicon-germanium alloys.
In the above process, cleavage is obtained by means of heat treatment, but it is has been proposed, in variants of the method, to bring about cleavage by that kind of heat treatment and/or by applying a splitting stress (for example, by inserting a blade between the two substrates and/or by applying further traction and/or bending and/or shear forces and/or by further application of ultrasound or microwaves of judiciously chosen power and frequency).
It may be noted that the above method implies the formation of a weakening layer formed of microcavities resulting from gaseous species ions and that in practice it is light ions that are implanted. Implanting light ions has the advantage that implantation to significant depths may be easily effected, but this is counterbalanced by the fact that these ions create few defects in the material so that it is therefore necessary to introduce them in large quantities to induce sufficient weakening (typically greater than 1016 or even 1017).
The paper “Ion implantation into GaN” by S. O. Kucheyev, J. S. Williams, S. J. Pearton in Materials Science and Engineering, 33 (2001) 51-107 and the paper “Ion implantation in GaN at liquid-nitrogen temperature: Structural characteristics and amorphization” by C. Liu, B. Mensching, M. Zeitler, K. Volz and B. Rauschenbach in Physical Review B of The American Physical Society, 1998, vol 57, No 4, pp. 2530-2535, disclose that implanting Au or Ar ions in GaN has various advantages, in particular that it enables a well-defined impurity to be implanted at a well-defined depth in a substrate; in particular, it is therefore possible to effect precise doping or to insulate precise volumes electrically or to cause the localized formation of appropriate inclusions. However, experiments have been carried out to characterize the deterioration induced by this kind of implantation and to determine how to eliminate or at least minimize it.
One effect of implanting gold ions in GaN is the creation of crystalline disorder associated with the formation of microcavities that consist of “bubbles” of N2 that result from the agglomeration of N atoms from the substrate. The passing implanted ions cause cascaded displacements of substrate atoms. These cascaded collisions induce a stoichiometric imbalance in the substrate: the heavier element (Ga in the case of GaN) is in excess at shallow depths while the deeper regions are enriched with the lighter element (N in the case of GaN). This effect increases in proportion to the mass of the ions implanted.